Create a session template with the busses you want, then right click on the track and create new aux send to that bus. Not quite as smooth, but very close honestly. I think the aux sends can even be part of the template so you get something VERY close indeed if you wanted.

Mixbus has a fixed and limited number of Busses, that's key for a fixed layout with dedicated bus-assignments.
The solution is simple: If you like dedicated bus-assignments use Mixbus, if not or if need variable-channel and flexible busses, use Ardour.

mrskytown: I find the mixbus layout limited and busy. I like some of the things in Mixbus such as the DSP being built in and therefore having a nice UI. However, building the same limitations as a hardware mixer into software doesn't really make sense. The hardware layout even when the physical size is much bigger than my screen would still be busy (but learnable) but shrinking to fit on a computer screen when the UI has many more possibilities doesn't make sense to me. For example, the Mixbus channel strip "wastes" a lot of space on routing using 8 or 12 buttons/pots of space that could just as easily be done with a dropdown for something that gets used once in any one session... oh, Ardour actually does this... and in doing so leaves more room for plugins.

Faders get used much more often and so are bigger. In hardware a fader knob is required so fingers can manipulate it, but in software a simple line makes it easier to see where the level really is and does not make the user think they must click on the knob to move it. Rather, the mouse can use any part of the fader to move it. A much "nicer" user experience... less time aiming the mouse, more time adjusting without thinking too much. Buttons are sized by frequency of use and buttons that have more modes than on/off have dropdowns rather than 4 buttons (automation for example).

Two things I would like to see, but that go somewhat deeper than just GUI design, are mute groups and scenes. I think at least mute groups would be possible with a script. Both could be done with a smart enough control surface as well.

Ardour (and Mixbus) offers the ability of having a simpler (or more complex) UI with the use of OSC for remote control on a tablet or hardware surface. Such a UI is totally customizable and the user could even do it with a web interface and make it look like anything they want with any layout they want. However, having done some of this, I would be one of the first to point out that the GUI is actually quite good and trying to create something that even does as much is hard.

So Mixbus is a great layout for people who have been working under the constraints of hardware driven mixing and want an easy to learn digital solution. It has proved very popular and it looks "finished" to someone used to using hardware. Because Mixbus is based on Ardour, it is possible to add more buses (though of course they look like input tracks) and use sends to route to them... but at that point it becomes easier to use Ardour.